In a Doppler radar transmitting pulses with a repetition frequency f.sub.R, the transmitted signal spectrum consists of a line at the carrier frequency and side lines located on either side of the carrier frequency at intervals equal to the repetition frequency. The received signal undergoes relative to the transmitted signal a delay equal to the time taken by the latter to cover the distance from the radar to the target and back to the radar plus a frequency shift due to the Doppler effect.
The same antenna is used for transmission and reception. The receiver is gated off during the transmission time. There will therefore be an eclipsing phenomenon, i.e. targets received at this time are blanked or "eclipsed". This phenomenon is the more bothersome as the form factor approaches 0.5.
The pulse-Doppler radars whose repetition frequency is high do not involve any velocity ambiguity but rather a range ambiguity due to the fact that one knows the signal delay received relative to the transmitted signal only with a modulus equal to the repetition period. This ambiquity over range can be removed through an adequate change in the repetition frequency.
A known method of measuring range on tracking operation consists in periodically modulating the radar transmission frequency. The spectrum of the received signal then consists of the main line and of side lines at the harmonics of the modulation frequency. The anplitude of such lines gives a measurement of the target range.
In this type of measurement, the width of the filters used for the various lines of the signal is constant. When the signal--to--noise ratio is constant, the accuracy of the measurement is also constant. However, a low signal--to--noise ratio does not allow accurate measurements.